Magnetic resonance imaging and spectroscopy at the nanoscale via probe paramagnetic centers

在这个由化学测量和成像计划资助的项目中，纽约城市大学(城市学院)的卡洛斯·A·梅里尔斯正在开发各种材料的磁共振成像(MRI)技术，包括活组织，其分辨率比目前可用的要高得多。核磁共振在诊断医学中是一项众所周知的技术，但它受到可可视化的体内结构大小的限制。最近的进展使显示单个细胞内的小细胞器成为可能，但研究人员仍然无法在这些细胞器内进行探测。这个项目寻求改进磁共振成像，以便在不使用侵入性手段的情况下获得越来越小的物体的清晰图像。本研究的重点是研制一种用于磁共振成像设备的新型探头。探测器由一个可以悬停在被研究样本上方的微小尖端组成，上面覆盖着一种特殊的钻石，其中两个相邻的碳原子已经被移除。氮原子取代了这些碳中的一个，但另一个碳被留作一个空洞。由此产生的缺陷被称为氮空位或NV缺陷，它通过改变样品的自转方式来响应被研究样品的变化。自旋方向的变化可以通过这个尖端在被激光照射后发出的光来检测。如果NV中心围绕指向上的轴旋转，则发射的光较强，但如果自旋围绕指向样品的轴向下旋转，则发射的光较弱。样本不同部分发射的光的差异可以输入计算机，并转换成分辨率比当前MRI图像高得多的图像。有一天，这项技术甚至可能允许科学家可视化蛋白质等大生物分子中的单个原子。因此，这项工作通过开发工具产生了广泛的影响，这些工具将在生物科学和医学中找到广泛的适用性。它正在产生更广泛的影响，通过在纽约城市学院以及伙伴机构的主办实验室举办暑期活动，为贫困学生提供参与这项研究的机会。该项目通过使用钻石中的氮空位(NV)中心来探索一种新的纳米尺度的自旋传感方式，从而解决了目前磁共振成像的局限性。该策略不是检测单个自旋，而是专注于NV中心与位于大约100立方纳米的有效体积上的小自旋系综相互作用的情况。在该小组最近在纳米尺度上观察到质子自旋噪声的基础上，目前的目标是通过新的协议来推进基于NV的自旋传感，该协议旨在增强观测信号的信息含量，并将该技术的适用性扩大到更一般的样本类别。这项工作有两个主要推力：(1)第一个推力区使用近表面的NVS来探测钻石表面的模型样品系统，其分子动力学通过诱导受控相变或通过限制扩散来改变。人们正在实施各种磁共振方案，以便通过光谱特征揭示样品分子的组成、迁移率和结构(如果可能的话)。(2)第二个主要推力涉及通过基于NV扫描尖端的几何学将重点从纳米级光谱学转移到纳米级成像。通过自上而下的纳米制造产生的一套独特的高纯度钻石纳米管正在与AFM共聚焦系统相结合，以展示具有纳米级空间分辨率的T1加权自旋成像。由于钻石主体中存在额外的顺磁缺陷不一定对传感有害，因此这项工作有另一个与第一个密切相关的目标，即探索替代方案，以初始化和控制广泛的工程纳米晶体中的自旋池。

In this project funded by the Chemical Measurement and Imaging program, Carlos A. Meriles of the City University of New York (City College) is developing magnetic resonance imaging, or MRI, techniques for a variety of materials, including living tissue, with much higher resolution than is currently available. MRI is a well-known technique in diagnostic medicine but it is limited in the size of structures within the body that can be visualized. Recent advances have made it possible to visualize small organelles within individual cells, but the ability to probe within these organelles still eludes researchers. This project seeks to refine MRI in order to acquire sharp images of ever-smaller objects without going to invasive means. The research is focused on the development of a new type of probe for the MRI device. The probe, consisting of a tiny tip that can be hovered over the sample being studied, is coated with a special kind of diamond in which two adjacent carbon atoms have been removed. A nitrogen atom replaces one of these carbons, but the other is left as an empty hole. The resulting defect, known as a nitrogen vacancy, or NV, defect, responds to variations in the sample being studied by changing the way it spins. The change in spin direction can be detected by light that is emitted from this tip after it has been illuminated by a laser. The emitted light is stronger if the NV center spins around an axis pointing up, but it is weaker if the spin is around an axis pointing down toward the sample. The difference in emitted light in different parts of the sample can be fed into a computer and converted into an image that has a much higher resolution than current MRI images. It is possible that one day this technique may even allow scientists to visualize single atoms within large biological molecules such as proteins. This work is, thus, having a broad impact through the development of tools that will find wide applicability in biological science and medicine. It is having a further broad impact through the development of opportunities for underprivileged students to participate in this research through summer activities at the City College of New York as well as in the host laboratories of partner institutions. The project addresses a current limitation of MRI by exploring a new modality of spin sensing at the nanoscale via the use of a nitrogen vacancy (NV) centers in diamond. Rather than detecting single spins, the strategy focuses on the case where the NV center interacts with small ensembles of spins localized over effective volumes of about a hundred cubic nanometers. Building on the group's recent observation of proton spin noise at the nanoscale, the current goal is to advance NV-based spin sensing via new protocols designed to enhance the information content of the observed signals and broaden the technique's applicability to a more general class of samples. The work has two main thrusts: (1) The first thrust area uses near-surface NVs to probe model sample systems on the diamond surface whose molecular dynamics is changed by inducing a controlled phase transition or by restricting diffusion. Various magnetic resonance schemes are being implemented so as to expose the composition, mobility and, if possible, the structure of the sample molecules via spectroscopic signatures. (2) The second main thrust involves a shift in emphasis from nanoscale spectroscopy to nanoscale imaging via a geometry based on an NV-hosting scanning tip. A unique set of high-purity diamond nanopillars produced via top-down nanofabrication is being combined with an AFM-confocal system to demonstrate T1-weighted spin imaging with nanoscale spatial resolution. Since the presence of additional paramagnetic defects in the diamond host is not necessarily detrimental to sensing, the work has another goal, closely related to the first, to explore alternate protocols conceived to initialize and control the spin bath in a broad set of engineered nanocrystals.